Process of producing absorbent paper



Patented July 27, l93 7 UNITED STATES PATENT OFFICE PROCESS OF PRODUCING ABSORBENT PAPE ware

No Drawing. Application August 1, 1934, SerialNo. 738,018

2 Claims.

consistency higher than usually used in the caustic treatment of pre-liberated pulp at low temperature. The present invention is designed to produce a product of substantially the same characteristics but by means of the difierent process involving pressure cooking with weak alkali at pulp consistencies lower than usually used in production of absorbent paper.

cooking processes, the conditions of temperature,

alkali strength, pulp consistency, and duration of the treatment being carefully regulated so as to produce a pulp having the required curl and crinkle as measured by the increase in freeness of the pulp.

An object of this invention is to provide a process for crinkling and curling chemical wood pulp fibers. Another object is a process for the Still another object is the production of paper suitable for impregnation with drying oils, natural or syn-, thetic resins, cellulose esters or ethers, rubber,

and the like. Other objects will-appear hereinafter.

These objects are accomplished by treating chemical wood pulp with dilute alkali solution at elevated temperature and under certain specific conditions more fully disclosed below, and forming the treated pulp into sheet material.

The initial material in the present process which is subjected to our specific caustic treatment is a chemical paper pulp which has been produced by any of the usual processes yielding a fully cooked pulp; If sulfite liquor has been used as the cooking medium this should be washed out until the pulp is practically free from the liquor. Kraft pulp made by the well known sulfate process responds best to the treatment of the present invention.

The cooking liquor used in our treatment of the pre-liberated paper pulp just mentioned is preferably sodium hydroxide or sodium carbonate. It is necessary that the concentration of this liquor be held within rather definite limits or otherwise the production of a curled fiber suitable for making an absorbent sheet will not be accomplished. The preferred concentration is 5 from about 2% to about 4%, although it is possible to operate with some success between the limits of 0.5% and 7.0%. p

The pulp consistency, that is, the ratio of liquor to the pu p treated, is also an important factor in our process. The pulp is dispersed in the dilute solution of the alkali at a fiber consistency which permits freedom of movement for the individual fibers of the pulp. The best results are obtained at fiber consistencies between 2.5 per cent and 4.5 per cent. For economical reasons it is desirable to operate at as high a fiber consistency as possible consistent with satisfactory freedom of movement of the fibers.

The temperature at which the dispersion of fiber in the dilute alkali is cooked should be in. the range of C. to C. The temperature in any instance should not be much higher than 200 C. nor much lower than 160 C. If the temperature is too low the fibers are not curled and crinkled, and if the temperature is too high the fibers are seriously degraded by the alkaline solution. The pulp suspension is kept agitated during the cooking operation. This is best accomplished by rotating the pressure cooker, although a stirring device within the cooker may be used to good advantage.

It is essential also that the duration of the pressure cooking be held within certain limits. This is determined by the time required to produce the optimum increase in freeness and the optimum curl and crinkle of the fiber, The increase in freeness, which is determined in the manner described in the mentioned Patent 1,857,100, should not be less than 20%. The cooking time corresponding to these requirements is usually about 2 hours and seldom less than hour or more than 3 hours. Too short a cook will not give the desired curling and crinkle, and too long a cook will seriously degrade the fibers.

To obtain a good product of uniform quality it is necessary that good agitation during the digestion process be provided. In operating on the small scale it has been found that low pulp consistencies make it easier to maintain good agitation. On the large scale good agitation is more readily obtained and higher pulp consistencies are employed with satisfactory results. In general the preferred pulp consistency is the maximum consistency permitting satisfactory agitation in the digester under the conditions of operation.

The duration of the digestion treatment is also important. .In this matter-the requirement is similar to that disclosed in U. S. P. 1,857,100, but is distinguished from the process of the patent in that the time is in general somewhat longer due to the slower reaction at the high temperature. During the digestion treatment the freeness of the pulp increases but eventually reaches a maximum and thereafter gradually decreases. In operating our process therefore we continue the digestion at the selected temperature until the freeness is at least 20% above that at the start, and we discontinue the treatment before the figure for the percentage freeness increase falls below 20%.

The treatment of chemical wood pulp under the conditions specified above causes the fibers to assume a permanent curl and crinkle. Before treatment the fibers are fiat and ribbon like in appearance. After treatment they are curled and crinkled, and somewhat more rod like in appearance. As a result of this change in the fiber a sheet formed from the treated pulp isporous, absorbent, pliable, and has improved resistance to paper break when bent. The sheet will absorb solutions or melts of materials that would remain essentially on the surface of a sheet formed from an untreated pulp. The result is uniform impregnation of the sheet which develops high sheet strength, and by the proper choice of impregnants the sheet becomes waterproof, moisture-proof, grease-proof, alkali and acid resistant and/or low in electrical conductivity, etc.

The following examples are illustrative of the method for carrying out our invention:

Example I 10 parts by weight of kraft pulp as wet lap (4-. parts of fiber and. 6 parts of water) is dispersed in 150 parts by weight of a 2.08 per cent water solution of sodium hydroxide. This dispersion is placed in an autoclave and heated as follows: hour up to 180 C., 2 hoursat 180 C., and 1 hour cool to 100 C. The autoclave is rotated during this cooking operation. The pulp is then removed from the autoclave and washed with water until neutral. The treated pulp is converted into sheet form by the following procedure: A beater is charged at approximately 4 per cent fiber consistency and the pulp is beaten for approximately 10 minutes with the beater roll set in a light brushing position. The pulp is then diluted to a paper making consistency and formed into sheets on a hand operated sheet mold. The sheet is then pressed between wool felts, and dried at 110 C.

A microscopic examination of the fibers shows that they have been substantially curled and crinkled by the cooking process. The treated pulp has a freeness of seconds, compared with 80 seconds for the untreated pulp. A description of the freeness tester used for these measurements is given in U. S. 1,857,100.

The sheet formed from the treated pulp is pliable, porous, absorbent, and has improved freedom from paper break. The high strength of the sheet when impregnated with rubber is illustrated by the following data. An impregnated sheet weighing 35.5 g. per 10" by 10" area, having a thickness of 0.052", and containing 40 per cent rubber from rubber latex has an El'mendorf tearing strength of 1410 g., and excellent resistance to hand tear.

Example II given under Example I is followed, and essentially the same results are obtained.

Example III 10 parts by weight of bleached sulfite pulp as wet lap (4 parts of bleached sulfite fiber and 6 parts of water) is substituted for the 10 parts of kraft wet lap used under Example I. The procedure given under Example I is followed.

The treated pulp has a freeness of 120 seconds as compared with 200 seconds before treatment. Sheets formed from the treated pulp are porous, absorbent, pliable, and have improved freedom from paper break.

Although we prefer to use sodium hydroxide or sodium carbonate for treatment of the fibers, we have found that other alkalies such as sodium meta-silicate, tri-sodium phosphate, or ammonia may be used. The addition of certain salts such as sodium chloride increases the effectiveness of the alkali solution. The solution of alkali may contain sodium chloride in any amount up to a saturated solution. The alkali solution may be modified with wetting agents, such as sulfonated oils or sulphonated higher alcohols, etc.

The process is not limited to the treatment of pulps that have been prepared by a particular adaptation of the kraft or sulphite cooking processes. Foreign or domestic kraft or sulphite pulp may be used. Mechanical wood pulp treated by the process of this invention also has in sheet form improved absorbency and pliability.

The process described herein may be applied to the crinkled'fibers which are made by the process set forth in the mentioned Patent 1,857,- 100. In this manner there is produced a pulp the fibers of which are crinkled and curled to an unusually high degree. This doubly treated pulp when made into a sheet yields a product having an exceedingly high porosity, absorbency. pliability 'and softness.

The pulp prepared by our process may be treated before sheet formation by procedures well known to the art with materials such as soaps and alum for water-proofing, wax emulsions, starch, dyes, filling pigments such as clay or calcium carbonate, and/or strengthening agents such as rubber latex. Broadly, any aqueous solution, dispersion, or emulsion may be added to the pulp followed by the addition of a precipitant therefor to deposit the substance in solution, dispersion, or emulsion onto the surface of the fibers in an insoluble form. Alum is the precipitant most commonly used in the art for depositing soaps, waxes, rubber from rubber'latex, asphalt,

etc., in an insoluble form onto the fibers before sheet formation.

The pulp after being treated by our process may be bleached with any of the common bleaching compositions such as calcium hypochlorite, or chlorine. Kraft pulp after the crinkling treatment bleaches to a pure white pulp and we are thus enabled to produce a white absorbent paper from kraft pulp.

The products of this invention are useful as an absorbent paper for impregnation with such materials as drying oils, varnishes, paints, lacquers, natural or synthetic resins, rubber, plasticized cellulose esters, etc. The impregnated products are useful as a base for coating with pyroxylin coating jellies or rubber, for use in book bindings, upholstery, suitcases, etc. The impregnated products are useful as floor coverings, wall coverings, insulating materials, shoe counters and toe caps, and other sundry uses to which said type of products are placed.

It will be apparent from the foregoing description that our invention provides a practical and economical process for the manufacture of an absorbent paper. An important advantage of our invention is that the caustic resulting after washing the strong caustic solution from the pulp treated by the process described in mentioned Patent 1,857,100 may be used in the present process. In the patented process a solution of approximately 10% sodium hydroxide, a part of which is reused and a part of which is recovered by evaporation, is obtained from the suction filters, and another solution of about 03% sodium hydroxide is obtained after washing out the residual caustic from pulp. The latter solution has heretofore been wasted because recovery of the caustic is not economically practicable. By mixing this 0.3% solution and the 10% solution which was sent to the evaporators, diluting when necessary, a sodium hydroxide solution of the strength required in our process is obtained. This utilization of the left over caustic lowers the product costs of both processes.

As many apparently widely difierent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments thereof except as described in the following claims.

We claim:

1. A process which comprises cooking liberated wood pulp under pressure at a temperature within the range of 170-190" C. with caustic soda solution of about 2-4% concentration and a pulp consistency within the range of 2.5% to 4.5%, continuing the treatment from one-half to not more than 3 hours until the freeness of the pulp has increased to at least 20% above that at the start and discontinuing the treatment before the percentage freeness increase falls below 20%.

.2. A process which comprises cooking liberated wood pulp under pressure at a temperature within the range of 170-190 C. with caustic soda solution of about 2-4% concentration and a pulp consistency within the range of 2.5% to 4.5%, continuing the treatment from one-half to not more than three hours until the freeness of the pulp has increased to at least 20% above that at the start, discontinuing the treatment before the percentage freeness increase falls below 20%, and felting the treated pulp into paper.

LOUIS LEONARD LARSON. GEORGE LEWIS scHwARTz. 

